Site Labeling of Proteins with Co(III)

The cobalt(III)-insulin hexamer is a prolonged-acting insulin prodrug

The insulin hexamer has two high-affinity metal ion binding sites, each involving three HisB10 residues, one from each dimer. Insulin hexamers containing Co2+ at both these sites were oxidized to form a stable Co(3+)-insulin complex. It is shown that the Co(3+)-coordinated insulin monomers are released extremely slowly in aqueous solution at pH 8.0, and that the hexamer does not spontaneously dissociate into subunits at nanomolar concentrations of insulin. The Co(3+)-insulin hexamer is not recognized by the insulin receptor in vitro but the complex shows a protracted action profile following subcutaneous (s.c.) injection into rabbits. The Co(3+)-insulin hexamer provides a novel prodrug approach to a soluble, prolonged-acting insulin preparation of potential use for basal insulin delivery in the treatment of diabetes.

Metal binding to yeast aminopeptidase I

Binding of Zn(II) and Co(II) to homogeneous dodecameric aminopeptidase I from yeast was studied. Apparent binding constants were estimated from the dependence of enzyme activity on metal levels in solution as established by metal buffer systems. The binding curves were sigmoidal with Hill coefficients of 1.2-1.6, depending on the metal and on pH. Metal concentrations at half-saturation were 28 nM with Zn(II), and 390 nM with Co(II) at pH 7.0, they increased with decreasing pH. Equilibrium binding studies yielded binding stoichiometries of 0.5-0.6 mol metal/mol subunit for both Zn(II) and Co(II). However, after oxidation of Co(II)-substituted enzyme with H2O2 almost stoichiometric amounts of cobalt (greater than 0.9 mol/mol subunit) were found. The oxidized enzyme was inactive and did not exchange cobalt with the solvent. Native aminopeptidase I was also affected by H2O2, however only after prolonged incubation and at a much lower rate. Apoenzyme modified by ethoxyformic anhydride could not be reconstituted by Zn(II) or Co(II). On the other hand, either metal afforded full protection against ethoxyformic anhydride. This finding, and the pH dependence of stability constants suggest that histidine residues are involved in metal binding. Both the reconstitution of active enzyme from apoprotein and metal and its inactivation due to loss of metal are slow processes with half-times in the minute range. The rate of reconstitution did not depend on Zn(II) concentration. The rates of metal loss were not enhanced by chelating agents containing carboxylate functions. In contrast, chelators coordinating via nitrogen atoms and 2,3-dimercaptopropanol accelerated dissociation. A model is proposed that accounts for the substoichiometric metal contents of aminopeptidase I and for the slow time course of reconstitution. It is assumed that reconstitution involves a slow, monomolecular transition, leading to an active enzyme conformation with lower affinity for metal ions.